The invention is directed to a boron containing precipitated silica as well as a process for producing this product.
It is known to produce boron containing silicon dioxide pyrogenically (see Kratel, German OS No. 2122066, the entire disclosure of which is hereby incorporated by reference and relied upon). However, this process of production of the flame hydrolytically recovered boron containing silicon dioxide is very expensive both technologically and industrially so that there has been sought for a long time to employ in its place a wet chemical method produced boron containing silicon dioxide, i.e., a so-called precipitated silica.
According to Nauroth German OS No. 2716225 it is possible to produce a boron containing precipitated silica by adding an aqueous, weakly acid boric acid solution to a washed, nearly electrolyte free precipitated silica filter cake, liquefying this mixture using shearing forces and spray drying, as well as grinding the spray dried product. The entire disclosure of the Nauroth German OS is hereby incorporated by reference and relied upon.
Although the process of production described in the Nauroth German OS No. 2716225 yields a qualitatively highly valuable product without additional expense for apparatus with the carrying out of the spray-drying which is increasingly employed in the preparation of precipitated silica itself, and in which Nauroth process the boric acid is in highly disposed form which is desired for use in polysiloxane-elastomers, there are substantial disadvantages to this process.
Thus, first according to the process of German OS No. 2716225 there can only be recovered boron containing precipitated silica whose B.sub.2 O.sub.3 content at maximum is 20 weight %. Because of the relatively low water solubility of ortho boric acid (at room temperature 4-6 weight %) after the paste fluidization spray drying process this B.sub.2 O.sub.3 content only can be attained if the precipitated silica filter cake is treated with large amounts of saturated ortho boric acid solution and there with the spray drying is undertaken from a relatively greatly diluted silica dispersion which unfavorably influences the cost of the drying process. Contents of boric acid of more than 20 weight %, however, also cannot be attained.
Furthermore in the process of German OS No. 2716225 there must be considered that the drying temperatures, particularly the exit dryer temperature of the spray dryer are exactly controlled and regulated since the loss occurring because of the the steam volatility of the ortho boric acid (in this regard, see H. Remy, Lehrbuch der Anorganischen Chemie, Vol. 1, page 400, 11th edition, 1960) leads to fluctuations in the B.sub.2 O.sub.3 content in the product. Also the ortho boric acid loss can be so high that the process is uneconomical if the temperature of the gases leaving the spray dryer goes above 110.degree.-115.degree. C.
Besides the steam volatile ortho boric acid can get into the free atmosphere with the dryer waste gases or with the inclusion of a so-called scrubber which can be connected downstream to a spray dryer it can get into the operating waste water which is detrimental to the environment.
The problem of the present invention was to develop a product and a process for its production which avoids the mentioned disadvantages.